The basic image-forming process of silver halide color photography comprises the exposure of a silver halide photographic recording material to actinic radiation (such as light) and the manifestation of a useful image by wet chemical processing of the material. The fundamental steps of this wet processing include color development to reduce silver halide to silver and to produce dye images in exposed areas of the material. During or after bleaching to oxidize metallic silver to silver(I), the silver(I) is generally removed by dissolving it in a silver(I) solvent, commonly known as a fixing agent. Conventional fixing steps generally require up to 6 minutes in large photoprocessing operations, and up to 2 minutes in small "minilabs" or small processing machines.
In some photochemical processes, bleaching and fixing are combined in a bleach-fixing step using a composition that includes both a bleaching agent to oxidize metallic silver and a fixing agent to dissolve the remaining silver(I).
A wide variety of fixing agents and silver solvents are known, as described for example in U.S. Pat. No. 5,633,124 (Schmittou et al) and publications noted therein. Thiosulfate salts are generally preferred as fixing agents because they are inexpensive, highly water soluble, non-toxic, non-odorous, and stable over a wide pH range. Thus, fixing is usually accomplished using a thiosulfate fixing agent that diffuses into the element, and forms silver thiosulfate complex that diffuses out of the element. In large photofinishing labs, the elements are usually immersed in a fixing solution for from 4 to 6 minutes. In small minilabs, the fixing time is shorter, that is from 90 to 120 seconds.
In processing some photographic elements, such as color negative photographic films, there is a need to reduce density from stain resulting from sensitizing dye aggregates formed from sensitizing dyes commonly included in the elements to increase silver halide spectral sensitivity. After the photographic elements are exposed, the spectral dyes are no longer needed, and the aggregates they form interfere with the absorption characteristics of the colored dyes that provide the final color images.
When using conventional fixing times, the unwanted dye aggregates disappear after the prescribed lengthy fixing and stabilizing (or washing steps). Substantial amounts of the sensitizing dyes remain in the color negative films after processing, but they are in invariant and unaggregated forms that absorb blue and green light. The absorbance by the retained unaggregated sensitizing dyes can be compensated for when final positive images are produced from the negative film images. However, when the fixing time is shortened, dye aggregates and resulting dye stains remain. This problem in the original image (such as color negative film images) is unacceptable in the photographic industry. It is also unacceptable for such images as color slides or transparencies, color prints or electronic images obtained from scanning original images.
It is well known that the rate of silver dissolution (or complexation) by thiosulfate fixing agents increases with increasing thiosulfate concentration until a maximum rate is reached. After this maximum rate is reached, the rate of silver dissolution decreases as the thiosulfate concentration is increased further. Consequently, other compounds are routinely incorporated into fixing solutions to act as co-fixing agents or fixing accelerators to improve silver removal. Thiocyanate is one of the most common compounds used for this purpose.
There are several fixing solutions available in the marketplace containing a combination of thiosulfate and thiocyanate for use in a 90-120 second fixing step. One such product is available as KODAK FLEXICOLOR RA Fixer Replenisher NR having thiosulfate and thiocyanate at 0.8 and 1.2 mol/l, respectively (1:1.5 molar ratio).
There are also numerous literature references to the combination of thiosulfate and thiocyanate including EP-A-0 610 763 (Buttner et al) that describes fixing for 90 to 240 seconds. However, this publication fails to appreciate the need to avoid sensitizing dye aggregate stains.
EP-A-0 712,040 (Ueda et al) describes the inclusion of organic sulfur-substituted compounds in fixing solutions that contain only thiosulfate. EP-A-0 189,603 (Rutges et al) describes the use of mercaptotriazole and thiosulfate in a combined fixing-stabilizing solution for processing silver halide materials containing at least 50 mol % silver chloride. EP-A-0 500,045 (Kojima et al) describes the use of mercaptoazoles as fixing agents.
U.S. Pat. No. 4,960,683 (Okazaki et al) describes the use of various heterocyclic thiols in thiosulfate fixers, in subsequent wash solutions, or in subsequent stabilizing solutions for the purpose of removing and washing out sensitizing dyes from black-and-white photographic materials. There is no mention of the action of such heterocyclic thiols on sensitizing dyes in silver halide color photographic systems, particularly aggregated sensitizing dye color negative film systems. The break-up of sensitizing dye aggregates into unaggregated forms and the elimination of aggregated sensitizing dye stain are the subjects of our invention.
There is a continuing need to provide images in photographic elements in a more rapid fashion. The industry is attempting to provide images to customers in less time, and thus a time reduction in any of the processing steps, including fixing, is highly desirable. The combination of thiosulfate and thiocyanate in conventional amounts is insufficient to complete the desilvering process in a more rapid fashion while eliminating unwanted dye stain from sensitizing dye aggregates.